Recently, a drive assist apparatus that supports a vehicle driving has been developed. This type of drive assist apparatus may evaluate a risk index using various types of sensor information when a subject vehicle follows a front vehicle, and then perform a drive assist (for example, notification of risk degree or brake control) based on the risk index. Conventionally, applicant of the present application proposed a performance index for approach and alienation as the risk index. The performance index for approach and alienation evaluates an approach state between the subject vehicle and the front vehicle (see, e.g., Patent Literature 1). The performance index for approach and alienation evaluates the approach and alienation state by setting a virtual front vehicle. With the configuration, speed control can be performed without giving discomfort to a user even when the front vehicle does not exist.